The present invention relates to structures of an apparatus, which performs transmission in a wireless communication system such as a cellular phone or a wireless LAN, and an integrated circuit which is used as a component in the apparatus. In particular, the present invention relates to a wireless communication apparatus for a wireless communication system which adopts a modulation system for transmitting information according to changes in both of a phase and an amplitude.
First, with reference to FIG. 7, a relation between input/output characteristics and a power efficiency of a power amplifier, which consumes largest electric power in a transmission apparatus for a wireless communication system and influences a power efficiency of the entire transmission apparatus, will be described (the figure is a schematic diagram and may not be always consistent with power efficiency characteristics of an individual power amplifier in some points). As it is seen from a curve 701 of the input/output characteristics, when an input power is increased while conditions of a circuit such as a bias voltage constant are kept, an output power of the power amplifier increases before a saturation point 702 in proportion to a log (dB) of the input power but becomes substantially constant regardless of the input power around or after the saturation point 702. A former region in which the output is proportional to the input is referred to as a linear region 703 and a latter region in which the output saturates to be constant is referred to as a saturation region 704. Linearity of the amplifier (a degree to which a transmission signal can be amplified while a form thereof is kept faithfully) is higher in the linear region. In the saturation region, since the form of the transmission signal is distorted largely, modulation accuracy and characteristics of spurious radiation to the outside of a communication channel band is deteriorated. On the other hand, a power efficiency 705 is higher in the saturation region. In the linear region, the power efficiency 705 is deteriorated rapidly as the input power falls. It is possible to vary a position and an input/output characteristic curve of the saturation point 702 by changing conditions of a peripheral circuit such as a bias voltage.
In a modulation system for transmitting information according to a change in a phase while keeping an amplitude of a transmission signal constant (e.g., BPSK: Binary Phase Shift Keying), distortion of an amplitude direction of the signal does not cause a problem. Therefore, in a wireless communication system using such a modulation system, it is a general practice to use a saturation region of an amplifier attaching importance to a power efficiency (hereinafter referred to as saturation amplifier). On the other hand, in a modulation system for transmitting information according to a change in only an amplitude or both of an amplitude and a phase (e.g., 16QAM: quadrature Amplitude Modulation), high linearity is required because data cannot be distinguished correctly on a reception side when a transmission signal is distorted. Therefore, in a wireless communication system using such a modulation system, it is a general practice to use a linear region of an amplifier attaching importance to linearity (hereinafter referred to as linear amplifier).
Here, with reference to FIG. 2, a structure of a transmission system wireless unit and an interface unit of a wireless communication terminal using a conventional linear amplifier will be described (see FIG. 1 and the section of the preferred embodiments of the invention about an overall structure of the terminal). A transmission IQ signal from a base band unit 201 is converted into an analog signal by DACs (Digital-to-Analog Converters) 203 and 204 in the interface unit 202 and subjected to filtering. Then, the signal is changed to a signal of a frequency in a desired frequency band by a quadrature modulator 206 in an RF-IC 205, subjected to gain adjustment conforming to transmission power control by an AGC (Automatic Gain Control) amplifier 207, amplified by a power amplifier 208, and then transferred to a front-end unit. In this structure, since the linear amplifier is used, it is easy to satisfy linearity but a power efficiency is low compared with the saturation amplifier.
A technique for increasing linearity by controlling an efficient saturation amplifier from the outside has also been studied. What is well known theoretically is an EER (Envelope Elimination and Restoration) system, an example of application of which to a wireless transmitter has been studied by F. H. Raab, et al. (F. H. Raab et al, “L-Band Transmitter Using Kahn EER Technique”, IEEE Trans. Microwave Theory Tech, vol. 46, pp. 2220–2225, De. 1998)
Next, with reference to FIG. 3, the principle of the EER system and a structure of the conventional transmission system wireless unit and interface unit using the EER system will be described. A transmission IQ signal 310 (801 and 802 in FIG. 8) from a base band unit 301 is separated into an amplitude component 311 and a phase component 312 (803 and 804 in FIG. 8) by an Rθ conversion section 305. More specifically, in this processing, a result obtained by synthesizing vectors of IQ components is divided into two. An amplitude of one component is limited by a limiter to be changed to a signal of a constant amplitude to include only phase information, and the other component is subjected to envelope detection to include only amplitude information. In this example, conversion processing is performed after changing a digital transmission signal into an analog signal by DACs 303 and 304. However, it is also possible to apply this processing to a digital transmission signal, and then convert the digital transmission signal into an analog signal. The phase component 312 is converted into a signal of a frequency in a desired frequency band by a mixer 307 in an RF-IC 306 and is inputted to a power amplifier 308. In this structure, since the saturation amplifier is used, an output has a constant amplitude regardless of an amplitude of an input signal (see 704 in FIG. 7). The amplitude component 311 is converted into a power supply voltage modulation signal of the power amplifier 308 by an amplitude modulation circuit 309 in the RF-IC 306 and is inputted to a power supply voltage pin of the power amplifier 308. By modulating a power supply voltage of the power amplifier, an envelope of an output waveform is changed to obtain a high frequency/high power output 313 having the same form as an original waveform.
With this system, since an efficient saturation amplifier can be used even in a wireless communication system including amplitude modulation, a highly efficient transmission system can be realized compared with the conventional system using a linear amplifier. However, as it is also apparent from FIG. 7, an amplifier deviates from the saturation region in a range in which an output power is low, and efficiency is deteriorated. In addition, since a dynamic range of an output power depends upon a variable range of a power supply voltage (range in which a change in an output power is proportional to a change in a power supply voltage), it is difficult to enlarge the range compared with the linear system in which a dynamic range is obtained by adjusting an input power. Therefore, this system has been considered unsuitable for a wireless communication system in which an output power changes sharply over a wide range, that is, a modulation system with a large PAPR (Peak to Average Power ratio) of a signal is used or an average level of an output power changes over a wide range by transmission power control.
In a cellular phone system of the second generation such as GSM (Global System for Mobile Communications), since a phase modulation system was mainly used, many wireless terminals used a saturation amplifier to obtain a power efficiency close to 60%. However, in a cellular phone system of the third generation and a wireless LAN system which have started service one after another in recent years, since a frequency usage ratio is required to be increased in order to cope with an increase in a data rate and an increase in the number of users, a multilevel modulation system which can transmit a larger amount of information at a time is often used. In the multilevel modulation system, since high linearity is required, many wireless terminals use a liner amplifier. However, a power efficiency thereof is 40 to 45% at the most, and an average power efficiency over all ranges of an output power is as low as 8 to 10%. Thus, an increase in power consumption is a problem. In particular, in a portable terminal driven by a battery, a reduction in power consumption (i.e., improvement of a power efficiency) is a significant object because it affects a continuous operation time. Therefore, a new technique has been required which reduces power consumption while satisfying a modulation accuracy and a provision of spurious radiation to the outside of a communication channel band.